This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-073487, filed Mar. 18, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a redundancy technology, and more particularly to a semiconductor device having fuses for laser repair.
With a further increase in the number of elements integrated on a semiconductor chip, some semiconductor devices employ a redundancy technology for saving a fault chip by replacing fault generated cell elements with spare elements.
For the switching to spare elements, a laser-based fuse cutting method is employed because of a smaller occupation area of a circuit associated therewith, and a relatively large degree of design freedom.
At present, fuses used for the redundancy technology are formed in a lower wiring layer, in which the fuses can be reduced in film thickness, for facilitating the cutting of the fuses by a laser. However, it is desired to form fuses in an upper wiring layer for reasons discussed below.
FIGS. 1A and 1B show a mutual relationship of occupation areas of fuses when the fuses are cut using a lower wiring layer and an upper wiring layer, respectively. In FIGS. 1A and 1B, reference numeral 81 designates a Si substrate; 82 an interlayer insulating film; 83 a lower layer wiring; 84 a fuse formed in the same layer as the lower layer wiring 83; 85 an upper layer wiring; and 86 a fuse formed in the same layer as the upper layer wiring 85.
As shown in FIGS. 1A and 1B, it can be seen that the fuse 84 formed in the same layer as the lower layer wiring 83 results in a significant limitation to an area available for forming other wiring in an upper layer above the fuse 84, as compared with the fuse 86 formed in the same layer as the upper layer wiring 85.
However, currently, the lower wiring layer 83 has a film thickness of approximately 300 nm or less, whereas the upper wiring layer 85 has a film thickness in a range of 400 nm to 1,600 nm, so that it is necessary to cut such thick fuses in order to form fuses in the upper wiring layer. Thicker fuses lead to problems such as a significant increase in laser energy required to cut the fuses, and more frequent occurrence of irradiation damages to regions other than a target fuse such as cutting a Si substrate, an adjacent fuse or the like.
In addition, with the miniaturization and higher integration of semiconductor devices, the miniaturization of fuses is also under progress. In order to employ miniature fuses, a shorter wavelength laser must be used to improve a focusing limit of a laser beam.
However, as shown in FIG. 2, the absorption coefficient of the laser beam for Si exhibits a significant increase in a region of wavelength below approximately 1,100 nm. In other words, a laser beam having a shorter wavelength resulting from a reduced beam diameter would cause a Si substrate underlying a fuse to be subjected to irradiation damages of the laser beam.
As described above, while a laser beam with a shorter wavelength has the advantage of improving the focusing limit of the beam diameter, it also has the disadvantage of a lower threshold energy with which the laser beam may cause an irradiation damage to a Si substrate. It is therefore necessary to provide a design for fuses with respect to the laser wavelength and beam diameter optimum for forming the fuses in a thick metallic film layer in deep consideration of such advantage and disadvantage as mentioned above. At present, however, no solution to the necessity has not been found.
As described above, fuses, when formed in an upper wiring layer, will have a larger film thickness. A thicker fuse leads to problems such as a significant increase in laser energy required to cut the fuse, and more frequent occurrence of irradiation damages to regions other than the fuse to be cut, such as cutting a Si substrate or an adjacent fuse.
On the other hand, while a laser beam with a shorter wavelength is advantageous in improving the focusing limit of the beam diameter, it also causes a problem of reducing the threshold energy with which the laser beam may cause an irradiation damage to a Si substrate.
It is an object of the present invention to provide a semiconductor device which is capable of reducing irradiation damages to regions other than a fuse to be cut, such as, for example, an underlying Si substrate or adjacent fuses other than the fuse to be cut, even when a thick fuse used for the redundancy technology is cut, or a fuse is cut using a laser with a wavelength of 1,100 nm or less.
To achieve the above object, a semiconductor device according to the present invention is configured in the following manner.
Specifically, in a semiconductor device of the present invention in which a redundancy fuse is formed above the lowest wiring layer, and the fuse is cut by irradiating with a laser having a wavelength in a range of 1,000 nm to 1,100 nm and a beam diameter D (xcexcm), the fuse has a film thickness T (xcexcm) and a width W (xcexcm) which satisfy:
Txe2x89xa6(xe2x88x920.15(D+2"sgr")+0.46) exp (2W)
Where "sgr" (xcexcm) is an alignment accuracy of the center of the laser beam to the center of the fuse (this designation of the symbol is also applied below).
In another aspect, in a semiconductor device of the present invention in which a redundancy fuse is provided which is cut by irradiating with a laser having a wavelength equal to or less than 600 nm and a beam diameter D (xcexcm), the fuse has a width W (xcexcm) which satisfies:
Wxe2x89xa7D+2"sgr"
Preferably, the fuse has a film thickness T equal to or larger than 400 nm.
Also preferably, the fuse is formed in a wiring layer other than the lowest wiring layer.
Further preferably, the fuse is formed in the top layer or in a layer immediately below the top layer.
The fuse is formed in the same layer as a wiring layer formed of a metal material such as Al or Cu, and the fuse is formed of the same metal material as the wiring layer in the same layer, formed of Al, Cu or the like.
With the above configuration, the present invention provides the following operations and effects.
In the case of a fuse to be cut using a laser having a wavelength in a range of 1,000 nm to 1,100 nm and a beam diameter D (xcexcm), the fuse may be formed to have a film thickness T (xcexcm) and a width W (xcexcm) which satisfy:
Txe2x89xa6(xe2x88x920.15(D+2"sgr")+0.46) exp (2W)
Where "sgr" (xcexcm) is an alignment accuracy of the center of the laser beam to the center of the fuse, with the result that the fuse can be cut without giving any irradiation damage to regions other than the fuse to be cut, such as a Si substrate or an adjacent fuse.
A laser having a wavelength in an infrared region from 1,000 nm to 1,400 nm is currently used for cutting a redundancy fuse. When a laser having a wavelength of 600 nm and a beam diameter D (xcexcm) is used to cut a fuse, the fuse may be formed to have a width W (xcexcm) which satisfies:
Wxe2x89xa7D+2"sgr"
with the result that the fuse can be cut without giving any irradiation damage to regions other than the fuse to be cut, such as a Si substrate or an adjacent fuse. Therefore, a beam focusing limit can be improved, and fuses formed at narrower pitches can be employed.
At present, fuses have a film thickness of approximately 300 nm or less since fuses are formed in a lower layer. However, a fuse having a film thickness of 400 nm or more can be cut under the foregoing equation. A fuse is formed in an upper layer, thus, area penalty due to the fuse can be reduced.
Also, while fuses are currently formed of the same material as a lower layer wiring which is made of such materials as silicon polycrystal or tungsten silicide, it is possible, according to the present invention, to cut a fuse made of Al, Cu or the like in an upper layer.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.